We propose to examine the communication, social behavior, and development of social behavior in the pygmy marmoset. The pygmy marmoset is potentially a very valuable species for behavior research due to its small size, ready adaptation to laboratory environments, social structure based on a nuclear family, extensive paternal care of infants, and relatively primitive evolutionary status. The animals are housed in a semi-naturalistic environment with approximately 250 cubic feet per "family" and trees, branches, and next boxes to mimic the natural environment. Three major areas would be studied: (1) development of a comprehensive behavioral inventory, recording the behavior patterns of normal social interactions and charting the development of these patterns from infancy. (2) recording and analyzing the vocalizations of the species. This would involve both a determination of the message of the call by carefully observing the behavior and context of the communicator and the meaning of the call through a similar study of recipients, but also analyzing the perceptual mechanisms by which the vocalizations are understood. This latter will be done through playback experiments of both natural and synthesized vocalizations. These experiments will be used to determine if there is individual specific information carried in a species-specific vocalization and to determine through systematic variation of the acoustical parameters of the vocalizations which parameters of the call are critical for the perception of the call. (3) studying the development of social behaviors: Because of the exclusive paternal care marmosets offer an ideal way of testing Harlow's notion that infant attachment is not related to secondary rewards deriving from nursing. The marmoset nurses on the female, but spends the rest of the time with male. Demonstrations of a social attachment which is stronger to the male than the female would indicate a dissociation of nursing and attachment without subjecting the animals to the socially disruptive effects of surrogate rearing.